MariaDB 5.5 and then MySQL 5.6 got Index Condition Pushdown (ICP) optimization (initially coded by yours truly). The idea of ICP is simple: after reading the index record, check the part of WHERE condition that can be computed using index columns, and only then read the table record. That way, we avoid reading table rows that don’t satisfy index condition:

It seems apparent that ICP can never make things slower. The WHERE clause has to be checked anyway, and not reading certain records can only make things faster.

That was what I thought, too, until recently Joffrey Michaie observed the contrary “in the wild”: we’ve got a real-world case where using Index Condition Pushdown was slower than not using it: MDEV-6713. The slowdown was about 20%, both on MariaDB and MySQL.

From what I could investigate so far, the slowdown is caused by these three factors working together:

a VARCHAR(255) column in the index. MySQL’s in-memory data representation for VARCHARs is not space efficient. If a column is defined as VARCHAR(255), any value will occupy the entire 255 bytes.

InnoDB’s row prefetch cache. When InnoDB detects that one is reading a lot of rows from an index, it pre-fetches index records and stores them in an internal cache. The cache uses the inefficient in-memory data representation.

Design of MySQL’s Item classes. One can’t evaluate an expression over a table record that is in InnoDB prefetch cache. Expression evaluation functions expect to find column values in the table’s “primary location”, internally known as table->record[0]. In order for ICP to check the condition, index columns have to be copied to table->record[0], first.

I hope we will be able to investigate this problem and post more about it soon. For now, the news is that ICP can cause a slowdown, when the index has big VARCHAR columns.